Paul,I don't ever use LR with videos, but since you mentioned it, I gave it a try. After selecting Import File/Video and while waiting for previews to be generated sure enough, I received the same error, and the video will import but not run, I'm not sure if this caused the error or I would have received it anyway...

I reported similar issues when Lightroom Classic CC was first released. Many other did the same. To my knowleged Adobe has not responded in an conclusive way. Myself and others have submitted requested information and logs with no result. In my case I am running a Macbook Pro with OSC High Sierra. The Adobe update was coincident with the Apple OSX update so we never know if it was related to that or otherwise. I do believe these issues have been reported on Windows machines as well but I am not 100% sure. Are you running Windows or Apple?

Please give your Mac the "once over"! Run Onyx (https://www.titanium-software.fr/en/onyx.html), (entire "Automation" tab, reboot). Not fixed? Run Disk Warrior (https://www.alsoft.com), if you can ($120), reboot. If you can’t buy Disk Warrior, at least run First Aid from Disk Utility. Also, try it from another (admin) account, as a test. Can you import from this other account into a new catalog? If you can, then maybe it's a preference in your yourusername/Library/ApplicationSupport/Adobe. Check The Lightroom Queen site (https://www.lightroomqueen.com) for instructions on how to reset your preferences. If that doesn't work you could try deleting that entire Adobe folder (reboot), or use App Cleaner to completely delete everything Adobe and reinstall it from scratch (Last Resort!)